This invention relates to nuclear magnetic resonance logging, and, more particularly, to a method and apparatus for magnetic resonance logging of an earth borehole to obtain information about properties of formations surrounding the earth borehole.
General background of nuclear magnetic resonance (NMR) well logging is set forth, for example, in U.S. Pat. No. 5,023,551. Briefly, in NMR operation the spins of nuclei align themselves along an externally applied static magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field (e.g. an RF pulse), which tips the spins away from the static field direction. After tipping, two things occur simultaneously. First, the spins precess around the static field at the Larmor frequency, given by xcfx890=xcex3B0, where B0 is the strength of the static field and xcex3 is the gyromagnetic ratio. Second, the spins return to the equilibrium direction according to a decay time T1, the spin lattice relaxation time. For hydrogen nuclei, xcex3/2xcfx80=4258 Hz/Gauss, so, for example, for a static field of 235 Gauss, the frequency of precession would be 1 MHz. Also associated with the spin of molecular nuclei is a second relaxation, T2, called the spin-spin relaxation time. At the end of a ninety degree tipping pulse, all the spins are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The net precessing magnetization decays with a time constant T2 because the individual spins rotate at different rates and lose their common phase. At the molecular level, dephasing is caused by random motions of the spins. The magnetic fields of neighboring spins and nearby paramagnetic centers appear as randomly fluctuating magnetic fields to the spins in random motion. In an inhomogeneous field, spins at different locations precess at different rates. Therefore, in addition to the molecular spin-spin relaxation of fluids, spatial inhomogeneities of the applied field also cause dephasing. Spatial inhomogeneities in the field can be due to microscopic inhomogeneities in the magnetic susceptibility of rock grains or due to the macroscopic features of the magnet.
A widely used technique for acquiring NMR data both in the laboratory and in well logging, uses an RF pulse sequence known as the CPMG (Carr-Purcell-Meiboom-Gill) sequence. As is well known, after a wait time that precedes each pulse sequence, a ninety degree pulse causes the spins to start precessing. Then a one hundred eighty degree pulse is applied to keep the spins in the measurement plane, but to cause the spins which are dephasing in the transverse plane to reverse direction and to refocus. By repeatedly reversing the spins using one hundred eighty degree pulses, a series of xe2x80x9cspin echoesxe2x80x9d appear, and the train of echoes is measured and processed.
Further background, set forth in the referenced copending parent application Ser. No. 08/936,892, is summarized as follows: The static field may be naturally generated, as is the case for the earth""s magnetic field BE. The NML(trademark) nuclear logging tool of Schlumberger measures the free precession of proton nuclear magnetic moments in the earth""s magnetic field. See, for example, U.S. Pat. No. 4,035,718. The tool has at least one multi-turn coil wound on a core of non-magnetic material. The coil is coupled to the electronic circuitry of the tool and cooperatively arranged for periodically applying a strong DC polarizing magnetic field, Bp, to the formation in order to align proton spins approximately perpendicular to the earth""s field, BE. The characteristic time constant for the exponential buildup of this spin polarization is the spin-lattice relaxation time, T1. At the end of polarization, the field is rapidly terminated. Since the spins are unable to follow this sudden change, they are left aligned perpendicular to BE and therefore precess about the earth""s field at the Larmor frequency fL=xcex3BE. The Larmor frequency in the earth""s field varies from approximately 1300 to 2600 Hz, depending on location. The spin precession induces in the coil a sinusoidal signal of frequency fL whose amplitude is proportional to the number of protons present in the formation. The tool determines the volume of free fluid in the formation. Additives in the borehole fluid are required to prevent the borehole fluid signal from dominating the formation signal. Also, there is necessarily a significant wait time before transients die down so that the coil can be used for detecting relatively small magnetic resonance signals.
A further nuclear magnetic resonance approach employs a locally generated static magnetic field, Bo, which may be produced by one or more permanent magnets, and RF antennas to excite and detect nuclear magnetic resonance (using, for example, the type of RF pulse sequence first described above), to determine porosity, free fluid ratio, and permeability of a formation. See, for example, U.S. Pat. Nos. 4,717,878 and 5,055,787.
As pointed out in the referenced copending Application, the tools and techniques developed in the prior art have various drawbacks that limit their utility in practical applications. These limitations include, among others, one or more of the following: a shallow depth of investigation, restrictions on the shape and size of the region of investigation, the need for treating of the borehole fluid, and the need for significant waiting between transmission and receiving.
It is among the objects of the present invention to address limitations of prior art nuclear magnetic resonance logging techniques and apparatus, and to devise improved logging methods and equipment for obtaining magnetic resonance characteristics of earth formations surrounding a borehole.
In the referenced copending U.S. patent application Ser. No. 08/936,892 there is disclosed an apparatus and technique for NMR logging that is based on non-resonant excitation and refocusing and exhibits a number of advantageous features: The volume of investigation is large compared with the conventional resonant operation. Also, the signal coming from different depths can be differentiated by its Larmor frequency. The technique thereof utilizes a pair of magnetic field generating sources, preferable orthogonally wound coils, that can be energized with large currents in a controlled manner to produce orthogonal magnetic fields in the formation. With appropriate switching of the currents, the direction of the generated magnetic field in the formation can then be changed abruptly. The rate of change of the direction of the magnetic field in the formation has to be fast compared to the local Larmor frequency. This way, the spins cannot follow the direction of the magnetic field and the spins end up orthogonal to the applied magnetic field. Effectively, it is as though all the spins have undergone a 90xc2x0 pulse. (In the conventional resonant excitation, only spins where the applied field is within a particular small range are excited. In practise, this leads to relatively thin shells of sensitive regions.) Now, the spins undergo a free induction decay with a Larmor frequency proportional to the local field produced by the presently activated coil. Since the field produced by the coil in the formation is highly non-uniform, there is a large range of Larmor frequencies and the net magnetization will decay very quickly (that is, T*2 is very short). This dephasing can be reversed by forming an echo which is achieved by reversing the field abruptly after a time t. The sense of rotation for the precessing spins is reversed and an echo is formed at a total time 2t, when the magnetization of all the spins is in phase again. This can then be repeated over and over to obtain a train of so-called gradient echoes.
Embodiments of the present invention also employ, inter alia, orthogonally oriented coils for transmission and detection, respectively, but do not use the just described technique of forming gradient echoes; instead using earth""s magnetic field as the operative static field. As in the above-summarized NML(trademark) tool, summarized in the Background portion hereof, a coil is used to apply a strong magnetic field, with a component perpendicular to earth""s magnetic field, that polarizes the spins, and when the applied field is turned off the spins precess around the earth""s magnetic field. In a form of the present invention, however, the same coil is not utilized for detecting the spins. Instead, the orthogonally oriented coil is utilized for detection. In an embodiment of this form of the invention, a third coil, preferably having its axis aligned with the tool axis, is used to create a magnetic field primarily in the borehole so that the spins therein precess at a frequency different from the frequency to which the detection system is tuned (namely, the Larmor frequency relatively deep in the formation). There is no need to chemically treat the borehole fluid. Also, in an embodiment of this form of the invention, a technique is used to reduce or eliminate spurious signals in the receiver coil caused by mutual inductance with the transmitting coil during its ring down. In this manner, detection can begin almost immediately after termination of the polarizing field, thereby increasing the sensitivity to spins with short relaxation times.
In accordance with an embodiment of the apparatus of the invention, equipment is provided for determining a nuclear magnetic resonance characteristic of formations surrounding an earth borehole. A logging device is movable through the borehole, the logging device having a longitudinal axis. A magnetic field generating means in the logging device is provided for generating a magnetic field in the formations. A magnetic field detecting means in the logging device is provided for detecting magnetic resonance signals from the formations, the magnetic field detecting means being separate from the magnetic field generating means. Means are provided for applying a polarizing signal to the magnetic field generating means. The magnetic field detecting means is operative, after the polarizing signal, to detect magnetic resonance of spins in the formations that are precessing around earth""s magnetic field.
In an embodiment of the apparatus of the invention, there is provided in the logging device a further magnetic field generating means, the further magnetic field generating means being operative to produce a further magnetic field in the borehole that reduces the contribution of spins in the borehole to the magnetic resonance detected by the magnetic field detecting means. In a form of this embodiment, the magnetic field generating means comprises a first coil and means for applying an energizing signal to said first coil, the magnetic field detecting means comprises a second coil that is oriented perpendicular to first coil, and the further magnetic field generating means comprises a third coil.
In accordance with another form of the invention, there is provided a method for measuring a nuclear magnetic resonance characteristic of earth formations surrounding a borehole, comprising the following steps: providing a logging device that is moveable through the borehole; producing, from the logging device, an audio frequency magnetic field in formations surrounding the borehole; reversing the phase of the audio frequency magnetic field in the formations; and detecting, at the logging device, a spin echo from the formations. In the preferred embodiment of this form of the invention, the reversing and detecting steps are repeated a number of times to detect a number of spin echoes from said formations, the reversals of phase being implemented at times xcfx84, 3xcfx84, 5xcfx84 . . . , and the spin echoes being detected at times 2xcfx84, 4xcfx84, 6xcfx84 . . . .
In accordance with another form of the invention, there is provided a method for determining a nuclear magnetic resonance characteristic of formations surrounding a borehole, comprising the following steps: (a) providing a logging device that is movable through the borehole; (b) producing, from the logging device, a magnetic pulse of a particular duration; (c) detecting, at the logging device, nuclear magnetic resonance signals resulting from the magnetic pulse of step (b); and, (d) repeating steps (b) and (c) for a number of further pulses having respectively different durations. In an embodiment of this form of the invention the magnetic pulses are pulses of audio frequency oscillations that are operative to tip spins in the formations that were initially aligned with earth""s magnetic field. In other embodiments, the pulses are adiabatic pulses ro composite pulses.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.